The introduction of CrystalEyes.RTM. eyewear in 1988 by StereoGraphics Corporation heralded the dawn of electronic stereoscopy as a viable medium for scientific visualization and other applications, as described in U.S. Pat. Nos. 4,884,876, 4,967,268, 5,117,302, and 5,181,133 to Lipton et al.
CrystalEyes.RTM. eyewear incorporates a shutter mounted in front of each eye, and these shutters, opening and closing in synchrony with the video field rate, alternately occlude and transmit left and right video fields. The shutters operate out of phase with each other, thereby transmitting the appropriate perspective view to one eye and blocking the inappropriate view from the other eye. If the fields are presented at a sufficiently high rate, the resultant stereoscopic image is perceived to be without flicker or flickerfree.
CrystalEyes.RTM. eyewear is often used in computer graphics applications with the above-and-below or subfield format described in U.S. Pat. No. 4,523,226. This format allows for the creation of stereopairs and doubles the field rate of the computer display. For example, if the computer runs at 60 fields per second, then using the above-and-below format, the total number of fields presented is increased to 120 per second. Thus, 60 fields per second are available for each eye and the result is perception of a flickerfree stereoscopic image.
According to the above-and-below approach, illustrated in FIG. 1, two subfields 101 and 103 are anamorphically squeezed in the vertical direction by a factor of two and are separated by a subfield vertical blanking interval 102. Subfield blanking interval 102 must have a vertical sync pulse (not shown) added to it which serves to index subfield 103 for the benefit of the display monitor.
In addition, this technique of tagging or indexing the subfield vertical blanking interval 102 lets the shuttering eyewear know which field has which perspective view. The adopted standard requires the left image to always be associated with the top subfield 101, which is immediately adjacent to the normal blanking area and sync pulse (not shown). Hence it is possible to unambiguously tag the left and right perspective views to the top subfield 101 and the bottom subfield 103, respectively. In the subfield approach, it does not matter whether scanning takes place in an interlace mode or in a progressive scan mode. The system is able to accommodate either approach.
Formatting images in this way has become widespread and is now used in many graphics computers, such as those manufactured by Silicon Graphics, Evans & Sutherland, Hewlett-Packard and others. By using the subfield approach, the manufacturer creates a format which is relatively inexpensive to implement because it has the same bandwidth as a planar format having half the number of fields. This system is dependent on having workstation monitors capable of running at approximately 120 fields per second, and there are now dozens of such monitors from which to choose. However, monitors are now a simple commodity, and there are literally hundreds of models of monitors. For the personal computer (PC) marketplace, as distinguished from the workstation marketplace, the ubiquitous multi-scan monitors typically run up to 90 fields per second, and no higher.
In the late 1970's and early 1980's, systems which ran at a total of 60 fields per second (or 30 fields per eye) were offered by Megatek for computer graphics applications and by Honeywell for video systems. A significant problem with this approach is that the user perceives tiresome flicker. Another problem is that the fields are not unambiguously tagged. Therefore, half of the time the image may be pseudostereoscopic instead of stereoscopic since the phase relationship of the shutters to the fields cannot be guaranteed. A pseudoscopic image is one in which the right eye sees the left fields, and vice versa. In such a case, the user must operate a control to put the shutters in phase with the fields so that a stereoscopic image rather than a pseudoscopic image is seen.
The pseudostereoscopic condition may be difficult to recognize because it is not seen in the real world. It will cause confusion because of conflicting stereoscopic and extrastereoscopic cues. It is an unpleasant effect which destroys the intention of the display to provide depth cues through binocular stereopsis.
Even though electronic stereoscopic displays of the flickerfree variety have a significant presence in the marketplace, there are still low-end display systems which provide only 30 fields per second per eye. These systems suffer both from flicker and from the ambiguous relationship of field to perspective view. As previously explained, this ambiguity does not exist with regard to the subfield or above-and-below technique.
The benefit of stereoscopic visualization, which has enhanced scientific and engineering applications for years, should also be enjoyed by users of low cost PC's. Thus, there is a need for a stereoscopic display to work in conjunction with commonly available PC graphics boards and generally available multi-scan monitors used in the PC marketplace. Such a system should have reduced flicker, and this can be achieved with almost any combination of personal computer, graphics board and multi-scan monitor. Such a system should function in both interlace and non-interlace modes, and it must have an unambiguous tag associated with one or both perspective views to insure the stereoscopic condition exists and preclude the pseudostereoscopic condition. Finally, it must be low cost because the PC marketplace is particularly price sensitive. Such a system is the subject of the present invention, and will be described below.